U.S. patent number 6,498,698 [Application Number 09/342,793] was granted by the patent office on 2002-12-24 for servowriter employing method of unlatching actuator arm using vcm voltage limiting circuit to limit actuator arm velocity.
This patent grant is currently assigned to Western Digital Technologies, Inc.. Invention is credited to Daniel M. Golowka, David M. Suckling.
United States Patent |
6,498,698 |
Golowka , et al. |
December 24, 2002 |
Servowriter employing method of unlatching actuator arm using VCM
voltage limiting circuit to limit actuator arm velocity
Abstract
A servowriter employs a method of unlatching an actuator arm
from a latch restraining the actuator arm in a head disk assembly
connected to the servowriter. The servowriter includes a first node
and a second node. The head disk assembly includes a voice coil
motor (VCM) coupled to the actuator arm. The VCM includes a coil
connected between the first node and the second node. The method
includes applying a voltage between the first node and the second
node to cause current to flow through the coil in order to move the
actuator arm away from the latch at an actuator arm velocity. The
method further includes temporarily activating a VCM velocity
control signal to enable a VCM voltage limiting circuit connected
in parallel with the coil between the first node and the second
node. The method includes limiting the voltage applied across the
coil to a predetermined VCM voltage level with the enabled VCM
voltage limiting circuit in order to limit the actuator arm
velocity.
Inventors: |
Golowka; Daniel M. (Rochester,
MN), Suckling; David M. (Rochester, MN) |
Assignee: |
Western Digital Technologies,
Inc. (Lake Forest, CA)
|
Family
ID: |
23343299 |
Appl.
No.: |
09/342,793 |
Filed: |
June 29, 1999 |
Current U.S.
Class: |
360/78.12;
360/75; G9B/5.222 |
Current CPC
Class: |
G11B
5/54 (20130101); G11B 5/59638 (20130101) |
Current International
Class: |
G11B
5/596 (20060101); G11B 005/596 () |
Field of
Search: |
;360/75,78.12,77.02,78.04 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Hudspeth; David
Assistant Examiner: Slavitt; Mitchell
Attorney, Agent or Firm: Shara, Esq.; Milad G. Sheerin,
Esq.; Howard H.
Claims
We claim:
1. A method of unlatching an actuator arm from a latch restraining
the actuator arm in a head disk assembly connected to a servowriter
that includes a first node and a second node, the head disk
assembly including a voice coil motor (VCM) coupled to the actuator
arm, the VCM including a coil connected between the first node and
the second node, the method comprising the steps of: a. applying a
voltage between the first node and the second node to cause current
to flow through the coil in order to move the actuator arm away
from the latch at an actuator arm velocity; b. temporarily
activating a VCM velocity control signal to enable a VCM voltage
limiting circuit connected in parallel with the coil between the
first node and the second node; and c. clamping the voltage applied
across the coil to a predetermined VCM voltage level with the
enabled VCM voltage limiting circuit in order to decelerate the
actuator arm toward a selected actuator arm velocity.
2. The method of claim 1 wherein the VCM voltage limiting circuit
includes a transistor having a voltage drop when the temporarily
activating step activates the VCM velocity control signal.
3. The method of claim 2 wherein the VCM voltage limiting circuit
includes a diode coupled in series with the transistor, the diode
having a forward voltage drop when the temporarily activating step
activates the VCM velocity control signal.
4. The method of claim 2 wherein the variable actuator arm velocity
is limited to a substantially constant actuator velocity when the
VCM voltage limiting circuit is enabled.
5. A servowriter connectable to a head disk assembly that includes
an actuator arm, a latch for restraining the actuator arm and a
voice coil motor (VCM) coupled to the actuator arm, the VCM having
a coil, the servowriter comprising: a first node; a second node;
the coil being connected between the first node and the second
node; a VCM driver for applying a voltage between the first node
and the second node to cause current to flow through the coil in
order to move the actuator arm away from the latch at a variable
actuator arm velocity; control means for temporarily activating a
VCM velocity control signal; and a VCM voltage limiting circuit
connected in parallel with the coil between the first node and the
second node, the VCM voltage limiting circuit being responsive to
the VCM velocity control signal being temporarily activated for
clamping the voltage applied across the coil to a predetermined VCM
voltage level in order to decelerate the actuator arm toward a
selected actuator arm velocity.
6. The servowriter of claim 5 wherein the VCM voltage limiting
circuit includes a transistor having a voltage drop when the
control means temporarily activates the VCM velocity control
signal.
7. The servowriter of claim 6 wherein the VCM voltage limiting
circuit includes a diode coupled in series with the transistor, the
diode having a forward voltage drop when the control means
temporarily activates the VCM velocity control signal.
8. The servowriter of claim 5 wherein the latch is a magnetic
latch.
9. The servowriter of claim 5 wherein the variable actuator arm
velocity is limited to a substantially constant actuator velocity
when the VCM voltage limiting circuit is enabled.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to servowriters. More particularly,
the present invention relates to a servowriter employing a method
of unlatching an actuator arm, in a head disk assembly, using a
voice coil motor (VCM) voltage limiting circuit to limit actuator
arm velocity.
2. Description of the Prior Art
A huge market exists for hard disk drives for mass-market host
computer systems such as servers, desktop computers, and laptop
computers. To be competitive in this market, a hard disk drive must
be relatively inexpensive, and must accordingly embody a design
that is adapted for low-cost mass production. In addition, it must
provide substantial capacity, rapid access to data, and reliable
performance. Numerous manufacturers compete in this huge market and
collectively conduct substantial research and development, at great
annual cost, to design and develop innovative hard disk drives to
meet increasingly demanding customer requirements.
In hard disk drives, data is stored on magnetic media disks in
concentric data tracks, which are divided into groups of data
sectors. Servo information is recorded in radially continuous
narrow wedges between the groups of data sectors. A head disk
assembly of a disk drive includes an actuator assembly having a
voice coil motor (VCM), an actuator arm extending from the VCM, and
a transducer head disposed at the end of the actuator arm. The VCM
includes a coil moving in proximity to a permanent magnet. The VCM
swings the actuator arm and the transducer heads back and forth
over the disk to access target data tracks on the disk surface.
During a latch mode, the transducer head is parked away from the
data tracks to protect the transducer head and the disk surface,
and a latch, such as a magnetic latch, typically restrains the
actuator arm in place in the head disk assembly. During an unlatch
mode, the VCM is controlled to overcome the force of the magnetic
latch to move the actuator arm away from the latch, in what is
referred to as unlatching the actuator arm, to position the
transducer head over the data area of the disk surface. The
actuator arm must be unlatched so that the transducer head can move
radially across the disk surface while the head disk assembly is
connected in either the disk drive or a servowriter. A problem
exits because the amount of current supplied to the VCM to break
free of the magnetic latch causes the actuator arm to move away
from the latch at a variable actuator arm velocity toward an outer
diameter crash stop in the head disk assembly or a push pin in the
servo writer. The actuator arm must be slowed to an actuator arm
velocity which will not cause damage to the transducer head or disk
surface during impact of the actuator arm against the outer
diameter crash stop or the push pin.
There exits substantial competitive pressure to develop mass-market
hard disk drives with more robust designs that are less sensitive
to operator handling and external vibrations. In particular, if the
actuator arm unlatches from the latch and the transducer head lands
into the data area of the disk, the transducer head and/or the disk
surface can be severely damaged. It is known to provide stronger
latches to resist the actuator arm coming off the latch when the
disk drive is in the latch mode, such as during the manufacturing
process or user handling of the disk drive. However, the increasing
strength of magnetic latches and other latches makes it more
difficult to unlatch the actuator arm, and higher unlatching
currents need to be applied to the VCM in order to unlatch the
actuator arm The increased unlatching currents result in a larger
variability of possible actuator arm velocities after the actuator
arm moves away from the latch, which increases the probability of
the actuator arm hitting the outer diameter crash stop or push pin
at too fast of an actuator arm velocity.
When the head disk assembly is placed in the servowriter for
writing servo information on the disks during the manufacturing
process, a first series of current pulses are applied to the VCM in
order to move the actuator arm away from the magnetic latch. A
second series of current pulses are then applied in the reverse
direction to reduce the actuator arm velocity. The amplitude and
widths of the second series of current pulses can be ascertained by
empirical analysis on a disk drive product. An example current
pulse sequence comprises an unlatch pulse period, a coast period,
and a brake pulse period. The unlatch pulse period typically
applies positive current to the VCM for a predetermined period of
time. The coast period is a predetermined period of time where zero
current is applied to the VCM. The brake pulse period typically
applies negative current to the VCM for a predetermined period of
time. For such a current pulse sequence, the predetermined time
periods and the amount of current applied to the VCM during the
unlatch pulse period and during the brake pulse period must have
sufficient margins to allow for variations in the VCM and latch
assembly of the particular disk drive. Moreover, if the negative
current applied to the VCM to slow the actuator arm during the
brake pulse period is too high or if the negative current is
applied for too long of time period, the actuator arm may return to
an inner diameter crash stop and become re-latched in the
latch.
When the head disk assembly is connected in the disk drive, a
similar current pulse sequence is used to unlatch the actuator arm
from the magnetic latch. However, the brake pulse period is
typically employed to slow down the actuator arm velocity to a
sufficiently slow velocity to permit a servo system in the disk
drive to detect the servo information on the disk surface. Once the
servo system of the disk drive is able to detect the servo
information, the servo system employs conventional closed loop
servo control to control the actuator arm velocity and the position
of the actuator arm over the disk. If the servo system, however,
does not detect the servo information, the actuator arm velocity
can not be controlled by the closed loop servo system. If the
actuator arm velocity is not controlled by the closed loop servo
system, the actuator arm velocity when the actuator arm hits the
outer diameter crash stop can be at a level which causes damage to
the transducer head and/or the disk.
A disk drive that employs a ramp load design may include a velocity
feedback circuit to control the actuator arm velocity. The velocity
feedback circuit employs the back electromotive force (BEMF) of the
VCM to monitor and control the actuator arm velocity to thereby
control the speed at which the transducer head moves away from the
ramp and over the disk surface. The velocity feedback circuit
includes complex closed loop circuitry to measure the BEMF and
compute actuator arm velocity that is used for controlling the
amount of current applied to the VCM.
For reasons stated above, there is a need for a circuit or method
to control the actuator arm velocity in a cost effective manner in
a head disk assembly of a hard disk drive after the actuator arm is
released and moves away from the latch.
SUMMARY OF THE INVENTION
The invention can be regarded as a method of unlatching an actuator
arm from a latch restraining the actuator arm in a head disk
assembly connected to a servowriter. The servowriter includes a
first node and a second node. The head disk assembly includes a
voice coil motor (VCM) coupled to the actuator arm. The VCM
includes a coil connected between the first node and the second
node. The method includes applying a voltage between the first node
and the second node to cause current to flow through the coil in
order to move the actuator arm away from the latch at an actuator
arm velocity. The method further includes temporarily activating a
VCM velocity control signal to enable a VCM voltage limiting
circuit connected in parallel with the coil between the first node
and the second node. The method includes limiting the voltage
applied across the coil to a predetermined VCM voltage level with
the enabled VCM voltage limiting circuit in order to limit the
actuator arm velocity.
The invention can also be regarded as a servowriter connectable to
a head disk assembly that includes an actuator arm, a latch for
restraining the actuator arm and a voice coil motor (VCM) coupled
to the actuator arm The VCM has a coil. The servowriter includes a
first node and a second node. The coil is connected between the
first node and the second node. The servowriter includes a VCM
driver for applying a voltage between the first node and the second
node to cause current to flow through the coil in order to move the
actuator arm away from the latch at a variable actuator arm
velocity. The servowriter further includes control means for
temporarily activating a VCM velocity control signal. The
servowriter includes a VCM voltage limiting circuit connected in
parallel with the coil between the first node and the second node.
The VCM voltage limiting circuit is responsive to the VCM velocity
control signal being temporarily activated for limiting the voltage
applied across the coil to a predetermined VCM voltage level in
order to limit the actuator arm velocity.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a hard disk drive employing a method
of unlatching an actuator arm using a VCM voltage limiting circuit
to limit the actuator arm velocity.
FIG. 2 is a block diagram of a head disk assembly (HDA) placed in a
servowriter employing a method of unlatching an actuator arm using
a VCM voltage limiting circuit to limit the actuator arm
velocity.
FIG. 3 is a more detailed diagram of an HDA that is connectable in
the disk drive of FIG. 1 or in the servowriter of FIG. 2.
FIG. 4 is a schematic diagram of the VCM voltage limiting circuit
employed in the servowriter of FIG. 2.
FIG. 5 is a schematic diagram of the VCM voltage limiting circuit
employed in the disk drive of FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Disk Drive
Referring to FIG. 1, a hard disk drive 30 employs a method of
unlatching an actuator arm, such as an actuator arm 62, using a VCM
voltage limiting circuit, such as VCM voltage limiting circuit 302,
to limit velocity of the actuator are.
Disk drive 30 includes a disk controller circuit board 32 and a
head disk assembly (HDA) 34. Disk controller circuit board 32
includes circuitry and processors which provide an intelligent disk
control system interface between a host system 36 and HDA 34 for
execution of read and write commands. Host system 36 can include a
microprocessor based data processing system such as a personal
computer, or other system capable of performing a sequence of
logical operations. Host system 36 includes a standard power supply
38 which supplies power to disk controller circuit board 32 via a
power supply connector 40. Data is transmitted between host system
36 and disk controller circuit board 32 via a host bus connector
42.
HDA 34 includes an actuator assembly 44, a preamplifier 46, and a
disk assembly 48. Disk assembly 48 includes a plurality of magnetic
media disks, such as indicated at 50. Disks 50 are stacked on a
spindle assembly 52. Spindle assembly 52 is mechanically coupled to
a spindle motor 54 for rotating disks 50 at a high rate of speed.
Each disk 50 includes two disk surfaces capable of storing data
thereon, such as indicated at 56 and 58. Actuator assembly 44
includes a voice coil motor (VCM) 60 and actuator arms 62 extending
from VCM 60. Each actuator arm 62 corresponds to a respective disk
surface such as 56 or 58. A transducer head 64 is disposed at the
end of each actuator arm 62, and each transducer head 64 is
associated with a disk surface 56 or 58. Transducer heads 64
communicate with disk controller circuit board 32 via preamplifier
46 for reading and writing data to the transducer head's associated
disk surface. Preamplifier 46 is electrically coupled to transducer
head 64 for receipt and amplification of position signals
representative of the position of transducer head 64. Preamplifier
46 provides an amplified signal to a read/write channel 68 of disk
controller circuit board 32. Read/write channel 68 performs
encoding and decoding of data written to and read from disks
50.
VCM 60 includes a coil 70 moving in proximity to a permanent magnet
72. Actuator arms 62 are coupled to VCM 60. VCM 60 swings actuator
arms 62 and their corresponding transducer heads 64 over their
associated disk surfaces 56 or 58 to access target data tracks
formed on the associated disk surface.
Disk control circuit board 32 includes a host interface and disk
controller (HIDC) integrated circuit 74. HIDC 74 includes a host
interface 76, a buffer controller 78, and a disk controller 80.
Host interface 76 communicates with host system 36 via host bus
connector 42 by receiving commands and data from and transmitting
status and data back to host system 36. Buffer controller 78
controls a buffer memory 82 employed for storing data from host
system 36 which is to be written to disks 50. In addition, buffer
controller 78 controls buffer memory 82 for storing read data from
disks 50 to be transmitted to host system 36 via host interface 76.
Buffer memory 82 typically comprises random access memory (RAM),
such as dynamic random access memory (DRAM).
Disk controller 80 sends data to and receives data from read/write
channel 68. Disk controller 80 also provides for error correction
and error detection on data transmitted to or read from disk
50.
An interface processor 84 handles the flow of data and commands
received by host interface 76 by sending commands to and reading
status from disk controller 80. Interface processor 84 ascertains
which commands to process from host system 36 and when to process
these commands, and directs other tasks performed by disk
controller 80.
A servo processor 86 commands a servo controller 88 to control the
position of transducer head 64 over disk 50 at a target data track
for subsequent execution of read or write commands. Servo processor
86 receives a representative form of the position signals sensed by
transducer head 64 and amplified by preamplifier 46 via read/write
channel 68 and servo controller 88 and performs calculations to
position transducer head 64 relative to its associated disk
surface. Servo processor 86 commands a digital-to-analog converter
(DAC) 90 in servo controller 88 to provide a corresponding analog
signal representing desired VCM coil current on a line 91 to a VCM
driver 92. VCM driver 92 responds to the analog signal on line 91
to provide a current to VCM 60. Power is supplied from standard
power supply 38 through power supply connector 40 to VCM driver 92
via a power line 99. More specifically, the current from VCM driver
92 is provided to coil 70 of VCM 60 and causes actuator arm 62 to
swing and thereby move transducer head 64 over an associated disk
surface 56 or 58 to access target data tracks.
Servo processor 86 also provides commands servo controller 88 to
control the rotational velocity of spindle motor 54. A DAC 94 in
servo controller 88 provides an analog signal to a spindle motor
driver 96. Spindle motor driver 96 responds to the analog signal
from DAC 94 to drive and thereby control the speed of spindle motor
54. Spindle motor driver 96 also detects a back electromotive force
(BEMF) of spindle motor 54 and provides a signal representative of
the spin-rate of spindle motor 54 to servo controller 88 which
converts the signal into a monitored velocity signal which can be
read by servo processor 86. In this way, servo processor 86 can
control the spin-rate of spindle motor 54 via servo controller 88
to maintain a substantially constant spin-rate of rotating disks
50.
Disk system operational programs are stored in non-volatile memory
97, such as read-only memory (ROM) or flash memory, and can be all
or partially loaded into RAM 98 for execution from RAM 98 or both
RAM 98 and non-volatile memory 97. Alternatively, portions of disk
system operational programs are stored on reserve cylinders on disk
50. Suitably, servo processor 86 may have integrated or separate
memory (not shown) for storage of servo programs.
The current track position of transducer head 64 is stored by servo
processor 86 to determine a required seek distance between the
current data track and a target data track. Based on the required
seek distance, servo processor 86 retrieves a corresponding read or
write seek profile and provides a digital signal command to DAC 90
corresponding to the seek profile. DAC 90 provides a corresponding
analog signal to VCM driver 92 representative of the seek profiles.
VCM driver 92 provides a current output to coil 70 of VCM 60 for
acceleration and/or deceleration of actuator arm 62 to perform a
seek operation to move transducer head 64 from the current data
track to the target data track. As actuator arm 62 moves from the
current data track to the target data track, position information
is received through the sensing of servo wedges disposed on disk
surface 56 or 58. Based on this position information, a position
error signal is provided via preamplifier 46, read/write channel
68, and servo controller 88 to servo processor 86 to provide a
representative position of transducer head 64 relative to its
associated disk surface 56 or 58. Upon completion of a seek
operation and the corresponding alignment of transducer head 64
over the target data track, a read or write command is executed to
read data from or write data to the target data track.
Servowriter
Referring to FIG. 2, HDA 34 is placed in a servowriter (servo track
writer) 134 employing a method of unlatching an actuator arm, such
as an actuator arm 62, using a VCM voltage limiting circuit, such
as VCM voltage limiting circuit 202, to limit velocity of the
actuator arm.
Servowriter 134 is employed for precisely writing servo tracks
(i.e., servo sectors) on disk surfaces 56/58 in HDA 34. Servowriter
134 writes a clock track 178 on disk surface 56, wherein the clock
track provides a continuous stream of reference timing information
to servowriter 134 during the servo writing process.
While positioned within servowriter 134 during the manufacturing
process, transducer head 64 communicates with servowriter 134 via
preamplifier 46 and a line 160 for writing servo sectors on the
associated disk surface 56. Preamplifier 46 receives and transmits
signals to and from a servowriter pattern generator 172 via a line
102.
Servowriter 134 includes clock head assembly 166, clock head
preamplifier and pulse detector (pulse detector) 168, phase
locked-loop (PLL) 170, pattern generator 172, positioning system
174, and spindle speed controller 176. Clock head assembly 166
initially operates to write clock track 178 on disk surface 56.
Clock assembly 166 also cooperates with pulse detector 168 and PLL
170 to provide a reference timing clock to pattern generator 172 by
reading the clock track 178. Pattern generator 172 operates to
provide servo information, for writing servo tracks, synchronized
to the reference timing clock during rotation of disk 50, which is
rotatably controlled by spindle speed controller 176. Positioning
system 174 operates to incrementally move the transducer head 64 in
an arcuate path from an outer diameter to an inner diameter of the
disk for writing each servo track.
Clock head assembly 166 includes clock arm 180 which extends over
disk surface 56. A clock head 182 is disposed at the end of clock
arm 180. Clock head 182 is electrically coupled to pulse detector
168 via connecting means 184. Clock head 182 communicates with
pattern generator 172 via pulse detector 168 and PLL 170 for
writing clock track 178 on disk surface 56 and subsequently reading
the clock track 178 for providing the reference timing clock to
pattern generator 172. Clock head preamplifier and pulse detector
168 receives signals from clock head 182 representative of magnetic
transitions which form the clock track 178 on disk surface 56.
Pulse detector 168 is electrically coupled to PLL 170 via
connecting means 186 and provides an amplified signal to PLL
170.
PLL 170 operates to lock onto the reference timing clock provided
by the clock track 178, and provides a reference timing clock to
pattern generator 172 which has been adjusted to compensate for any
speed variations in the rotation of disk 50.
Positioning system 174 provides for radial positioning of the
transducer head 64 during the servo writing process. In one
exemplary embodiment, positioning system 174 utilizes a laser
interferometer positioning system for precisely and incrementally
moving the transducer head 64 in an arcuate path from track to
track during the servo writing process.
Positioning system 174 includes a servo processor 186, a server
controller 188, a VCM driver 192, a motor driver 196, and a DC
motor 198. Servo processor 186 commands servo controller 188 to
control the position of the transducer head 64 over disk 50 during
the servo writing process. Servo processor 186 commands a DAC 190
in servo controller 188 to provide a corresponding analog signal
representing desired VCM coil current on a line 191 to VCM driver
192. VCM driver 192 responds to the analog signal representing
desired VCM coil current on line 191 to provide a corresponding DC
current to VCM 60. More specifically, the DC current from VCM drive
192 is provided to coil 70 of VCM 60 and causes actuator arm 62 to
swing and thereby bias actuator assembly 44 against a push-pin 106
of servowriter 134. Push-pin 106 is slideably mounted in a slot 107
during the servo writing process and is moved incrementally via
positioning system 174. Because actuator assembly 44 is biased
against push-pin 106, actuator arm 62 follows push-pin 106 and
incrementally moves from track to track.
Servo processor 186 also commands a DAC 194 in servo controller 188
to provide a corresponding analog signal to motor driver 196. Motor
driver 196 responds to the analog signal from DAC 194 to provide a
corresponding DC current to DC motor 198 which is mechanically
coupled to push-pin 106 via coupling means 199. In this way, DC
motor 198 is controlled by servo controller 188 to incrementally
move push-pin 106 to cause actuator assembly 44, which is biased
against push-pin 106, to incrementally move from track to
track.
Magnetic Latch and Crash Stops of HDA
Referring to FIG. 3, HDA 34 includes a crash stop plate 108 which
is fixedly fastened to HDA 34 with a screw 109. A magnetic latch
110 is attached to crash stop plate 108. Magnetic latch 110
includes an inner diameter crash stop 112 attached to crash stop
plate 108 and a permanent magnet 114 attached to inner diameter
crash stop 112. In one embodiment, inner diameter crash stop 112 is
integral to crash stop plate 108. A metal tab 116 is attached to
VCM 60. During a latch mode, magnetic latch 110 holds metal tab 116
with magnetic force to thereby restrain and hold actuator arms 62
at the inner diameter crash stop 112. During an unlatch mode,
current is provided to VCM 60 to release metal tab 116 from
magnetic latch 110 and move actuator arm 62 away from magnetic
latch 110 to thereby move transducer head 64 towards the outer
diameter of disk 50 at a variable actuator arm velocity.
When HDA 34 is connected to disk controller circuit board 32 in
disk drive 30, actuator arm 62 is stopped from moving past the
outer diameter of disk 50 by an outer diameter crash stop 118
attached to crash stop plate 108. In one embodiment, outer diameter
crash stop 118 is integral to crash stop plate 108.
When HDA 34 is placed in servowriter 134 during servo track writing
of disk 50, actuator arm 62 is stopped from moving past the outer
diameter of disk 50 by push-pin 106.
The below described embodiments refer to employing a VCM voltage
limiting circuit to limit actuator arm velocity, such as to a
substantially constant and safe actuator arm velocity, after
releasing and moving actuator arm 62 away from magnetic latch 110
in head disk assembly 34. However, the below described VCM voltage
limiting circuits can also be employed to limit the actuator arm
velocity after releasing and moving the actuator arm 62 from any
type of latch including, but limited to: a solenoid released spring
latch; a wind pressure released spring latch; or a ramp load latch.
For example, the below described VCM voltage limiting circuits are
useful with ramp load latches because as a transducer head flies
off the ramp, the actuator arm velocity needs to be controlled so
that neither the transducer head nor the disk surface is
damaged.
VCM Voltage Limiting Circuit in Disk Drive
Referring to FIGS. 1 and 5, disk drive 30 includes VCM voltage
limiting circuit 302 connected in parallel with coil 70 between a
first node 306 and a second node 308. Servo processor 86 commands
DAC 90 in servo controller 88 to provide the corresponding analog
signal representing desired VCM coil current on line 91 to VCM
driver 92. VCM driver 92 responds to the analog signal on line 91
to provide a corresponding current to VCM 60. More specifically,
VCM driver 92 is controlled to apply a voltage V (from power supply
38) between first node 306 and second node 308 to cause current to
flow through coil 70 of VCM 60 in order to release and move
actuator arm 62 away from magnetic latch 110 at a variable actuator
arm velocity, wherein the current has a magnitude that is
preferably determined by servo controller 88. In one embodiment,
VCM driver 92 is controlled so that current is provided to coil 70
of VCM 60 in the form of unlatch current pulses which typically
include forward and backward exciting current pulses to provide a
torque for releasing and moving actuator arm 62 away from magnetic
latch 110. Servo processor 86 then commands servo controller 88 to
temporarily activate a VCM velocity control signal on a line 304 to
limit the actuator arm velocity. VCM voltage limiting circuit 302
is responsive to the activation of the VCM velocity control signal
on line 304 for limiting the voltage V applied across coil 70 to a
predetermined VCM voltage level in order to limit the actuator arm
velocity to a substantially constant and safe actuator arm
velocity. VCM voltage limiting circuit 302 is preferably an open
loop circuit that is temporarily activated by the VCM velocity
control signal on line 304 in order to limit the actuator
velocity.
The VCM voltage limiting circuit 302 responds to the activation of
VCM velocity control signal on line 304 to provide a suitably low
resistance path in parallel with coil 70 between first node 306 and
second node 308 to limit the actuator arm velocity by limiting the
BEMF generated and applied across coil 70 of VCM 60. The voltage V
applied across coil 70 depends on the low resistance path in
parallel with coil 70. When VCM velocity control signal on line 304
is activated, the voltage V applied across the low resistance path
in parallel with coil 70 defines the predetermined VCM voltage
level across coil 70.
Thus, activating the VCM velocity control signal on line 304 to
provide the low resistance path in parallel with coil 70 of VCM 60
reduces and limits the actuator arm velocity to a substantially
constant actuator arm velocity. Actuator arm 62 decelerates until
the BEMF of VCM 60 is equivalent to the predetermined VCM voltage
level across coil 70 which is defined by the voltage V applied
across the low resistance path in parallel with coil 70 between
first node 306 and second node 308. At this point, actuator arm 62
is moving at a substantially constant and safe actuator arm
velocity toward outer diameter crash stop 118. The safe
substantially constant actuator arm velocity corresponds to the
predetermined VCM voltage level across coil 70 and can be
ascertained by knowing the BEMF factor or voltage constant (Ke) of
VCM 60 and the voltage Vcoil across coil 70 as shown by the
following Equations I:
V.sub.coil =V-BEMF=I.sub.coil *R.sub.t
Where: V.sub.coil is the voltage V less the BEMF across coil 70; V
is the voltage applied from power supply 38 via VCM driver 92
between first node 306 and second node 308 which is equal to (1)
the voltage applied from power supply 38 when VCM voltage limiting
circuit 302 is not enabled, and (2) the predetermined VCM voltage
level across coil 70 when VCM voltage limiting circuit 302 is
enabled; BEMP is the back electromotive force of the VCM;
I.sub.coil is VCM coil current; R.sub.t is the total resistance of
the VCM coil and the power FETs driving the VCM coil from the VCM
power source to ground; Ke is the Voltage constant of the VCM i.e.
the BEMF factor; and Vel.sub.actuator arm is the velocity of the
actuator arm coupled to the VCM.
Because VCM voltage limiting circuit 302 is controlled by control
means including servo processor 86 and servo controller 88 via the
VCM velocity control signal on line 304, VCM voltage limiting
circuit 302 can be temporarily activated at any time during the
unlatch mode, and the voltage V applied across coil 70 of VCM 60
can then be held at the predetermined VCM voltage level regardless
of the amount of current applied from power supply 38 via VCM
driver 92. Suitably, a low level current from power supply 38 is
still applied to coil 70 of VCM 60 even after the BEMF of VCM 60 is
equivalent to the predetermined VCM voltage level across coil 70,
which depends on the voltage V applied across the low resistance
path in parallel with coil 70. This low level current compensates
for any windage on disk assembly 48 and flex current bias in coil
70. In one embodiment, the substantially constant actuator arm
velocity is a sufficiently slow velocity to permit the VCM servo
system of disk drive 30 including read/write channel 68, servo
processor 86, and servo controller 88 to detect the servo
information on disk 50. Once the VCM servo system of disk drive 30
is able to detect the servo information, the VCM servo system
employs conventional closed loop servo control to control the
actuator arm velocity and the position of actuator arm 62 over disk
50.
Alternatively, the substantially constant actuator arm velocity is
a sufficiently slow velocity to permit actuator arm 62 to safely
contact outer diameter crash stop 118 without causing damage to
transducer head 64 or disk 50.
VCM Voltage Limiting Circuit in Servowriter
Referring to FIGS. 2 and 4, servowriter 134 includes a VCM voltage
limiting circuit 202 connected in parallel with coil 70 of VCM 60
when HDA 34 is positioned in servowriter 134. VCM voltage limiting
circuit 202 is connected between a first node 206 and a second node
208. Servo processor 186 commands DAC 190 in servo controller 188
to provide a corresponding analog signal representing desired VCM
coil current on line 191 to VCM driver 192. VCM driver 192 responds
to the analog signal on line 191 to provide a corresponding current
to VCM 60. More specifically, VCM driver 192 is controlled to apply
a voltage V (from an external power supply) between first node 206
and second node 208 to cause current to flow through coil 70 of VCM
60 in order to release and move actuator arm 62 away from magnetic
latch 110 at a variable actuator arm velocity, wherein the current
has a magnitude that is preferably determined by servo controller
188. In one embodiment, VCM driver 192 is controlled so that
current is provided to coil 70 of VCM 60 in the form of unlatch
current pulses which typically include forward and backward
exciting current pulses to provide a torque for releasing and
moving actuator arm 62 away from magnetic latch 110. Servo
processor 186 then commands servo controller 188 to temporarily
activate a VCM velocity control signal on a line 204 to limit the
actuator arm velocity. VCM voltage limiting circuit 202 is
responsive to the activation of the VCM velocity control signal on
line 204 for limiting the voltage V applied across coil 70 to a
predetermined VCM voltage level in order to limit the actuator arm
velocity to a substantially constant and safe actuator arm
velocity. VCM voltage limiting circuit 202 is preferably an open
loop circuit that is temporarily activated by the VCM velocity
control signal in order to limit the actuator velocity.
The VCM voltage limiting circuit 202 responds to the activation of
VCM velocity control signal on line 204 to provide a suitably low
resistance path in parallel with coil 70 of VCM 60 to limit the
actuator arm velocity by limiting the BEMF generated across coil 70
of VCM 60. The voltage V applied across coil 70 depends on the low
resistance path in parallel with coil 70. When VCM velocity control
signal on line 204 is activated, the voltage V applied across the
low resistance path in parallel with coil 70 defines the
predetermined VCM voltage level across coil 70.
Thus, activating the VCM velocity control signal on line 204 to
provide the low resistance path in parallel with coil 70 of VCM 60
reduces and limits the actuator arm velocity to a substantially
constant actuator arm velocity. Actuator arm 62 decelerates until
the BEMF of VCM 60 is equivalent to the predetermined VCM voltage
level across coil 70 which is defined by the voltage V applied
across the low resistance path in parallel with coil 70 between
first node 206 and second node 208. At this point, actuator arm 62
is moving at a substantially constant and safe actuator arm
velocity toward outer diameter crash stop 118 and push pin 106. The
safe substantially constant actuator arm velocity corresponds to
the predetermined VCM voltage level across coil 70 and can be
ascertained by knowing the BEMF factor (Ke) of VCM 60 and the
voltage Vcoil across coil 70 as shown by the above Equations I,
where V is the voltage applied from an external power supply via
VCM driver 192 between first node 206 and second 208 when using VCM
voltage limiting circuit 202 in servowriter 134.
Because VCM voltage limiting circuit 202 is controlled by control
means including servo processor 186 and servo controller 188 via
the VCM velocity control signal on line 204, VCM voltage limiting
circuit 202 can be temporarily activated at any time during the
unlatch mode, and the voltage V applied across coil 70 of VCM 60
can then be held at the predetermined VCM voltage level regardless
of the amount of current applied from the external power supply via
VCM driver 192. Suitably, a low level current from the external
power supply is still applied to coil 70 of VCM 60 even after the
BEMF of VCM 60 is equivalent to the predetermined VCM voltage level
across coil 70, which depends on the voltage V applied across the
low resistance path in parallel with coil 70. This low level
current compensates for any windage on disk assembly 48 and flex
current bias in coil 70.
The substantially constant actuator arm velocity is a sufficiently
slow velocity to permit the actuator arm 62 to safely contact push
pin 106 without causing damage to transducer head 64, disk 50, or
push pin 106.
Detailed Operation of VCM Voltage Limiting Circuit in
Servowriter
FIG. 4 is a detailed schematic diagram illustrating the operation
of VCM voltage limiting circuit 202 in servowriter 134 when HDA 34
is placed in servowriter 134. Servowriter 134 includes first node
206 and second node 208. Coil 70 of VCM 60 is coupled between node
206 and node 208. VCM driver 192 of positioning system 174 includes
a NPN bipolar transistor 210 coupled between a +12 volt power rail
and node 206 and includes a PNP bipolar transistor 212 coupled
between a -12 volt power rail and node 206.
VCM driver 192 includes a differential amplifier 219 that generates
a control signal 195 in response to (1) the analog signal
representing desired VCM coil current on line 191 and (2) a current
sense signal 193 that represents current flowing through a sense
resister 218 and coil 70. A resister 214 is coupled between control
signal 195 and a base of bipolar transistor 210. A resister 216 is
coupled between control signal 195 from and a base of bipolar
transistor 212. Control signal 195 controls transistors 210 and 212
to provide the desired VCM coil current through coil 70. Control
signal 195 causes one of transistors 210 or 212 to conduct current
from the +12 volts power rail through transistor 210 to node 206 or
current from the -12 volt power rail through transistor 212 to node
206.
According to one embodiment, VCM voltage limiting circuit 202
comprises a diode 220, a NPN bipolar transistor 222, a resistor
224, and a resistor 226. Diode 220 is coupled between node 206 and
a collector of bipolar transistor 222. An emitter of bipolar
transistor 222 is coupled to node 208. Resistor 226 is coupled
between a base of bipolar transistor 222 and a node 228. Resistor
224 is coupled between the +12 volt power rail and node 228. The
VCM velocity control signal from servo controller 188 is provided
on line 204 to node 228 to control the base of bipolar transistor
222. When servo processor 186 commands servo controller 188 to
temporarily activate the VCM velocity control signal on line 204,
bipolar transistor 222 goes from an off state to a saturated
condition which conducts current away from VCM 60 to limit the
voltage applied across coil 70 of VCM 60 to a predetermined VCM
voltage level in order to limit the actuator arm velocity, such as
maintaining the movement of the actuator arm at a substantially
constant actuator arm velocity.
Suitably, the predetermined voltage level across coil 70 of VCM 60
is the voltage drop from first node 206 to second node 208 which is
equal to the collector-emitter voltage drop (Vce) of bipolar
transistor 222 plus the forward bias voltage drop (Vfwd) of diode
220. In one example embodiment the Vce of bipolar transistor 222 is
approximately 0.2 volts and the Vfwd of diode 220 is approximately
0.7 volts resulting in a total voltage drop from node 206 to node
208 of approximately 0.9 volts.
Limiting the voltage applied across coil 70 to the predetermined
voltage level reduces the actuator arm velocity to a substantially
constant actuator arm velocity. Actuator arm 62 decelerates until
the BEMF of VCM 60 is equivalent to the predetermined VCM voltage
level between first node 206 and first node 208 which is equal to
Vce of transistor 222 plus Vfwd of diode 220.
At this point, actuator arm 62 is moving at a substantially
constant and safe actuator arm velocity toward outer diameter crash
stop 118 and push pin 106. The safe substantially constant actuator
arm velocity corresponds to the predetermined VCM voltage level
between first node 206 and first node 208 and can be ascertained by
knowing the BEMF factor (Ke) of VCM 60 and the voltage Vcoil across
coil 70 as shown by the above Equations I.
Alternatively, VCM voltage limiting circuit 202 does not include
diode 220 to thereby provide only the Vce of bipolar transistor 222
between nodes 206 and 208 once the VCM velocity control signal on
line 204 is activated. This alternative embodiment of VCM voltage
limiting circuit 202 is suitably employed in applications which
require a reduced substantially constant actuator arm velocity or
where the BEMF factor (Ke) of VCM 60 is quite low.
According to another alternate embodiment of VCM voltage limiting
circuit 202, diode 220 is replaced with a zener diode or a resistor
is coupled in series with diode 220 to increase the voltage drop
between nodes 206 and 208 once the VCM voltage control signal on
line 204 is activated. This alternate embodiment of VCM voltage
limiting circuit 202 is suitably employed in applications which
require an increased substantially constant actuator arm velocity
once the VCM velocity control signal 204 is activated or where the
BEMF factor (Ke) of VCM 60 is quite high.
The above described VCM voltage limiting circuit 202 is suitably
implemented in TTL logic using bipolar transistors for an
embodiment of servowriter 134 where VCM voltage limiting circuit
202 is implemented in discrete logic on a circuit board of
servowriter 134. An alternative embodiment of servowriter 134
implements a VCM voltage limiting circuit similar to VCM voltage
limiting circuit 202, but in CMOS logic using FETs in one of the
gate arrays mounted on a circuit board of servowriter 134.
Detailed Operation of VCM Voltage Limiting Circuit in Disk
Drive
FIG. 5 is a detailed schematic diagram illustrating the operation
of VCM voltage limiting circuit 302 in disk drive 30. Disk drive 30
includes first node 306 and second node 308. Coil 70 of VCM 60 is
coupled between first node 306 and second node 308. As illustrated
in FIG. 5, one embodiment of VCM driver 92 of disk drive 30
includes four FETs coupled in a conventional H-Bridge construction
across coil 70 of VCM 60. VCM coil 70 is coupled to the H-Bridge
FETs of VCM driver 92 between first node 306 and second node 308.
The H-Bridge FET construction of VCM driver 92 includes a P-FET 374
coupled between the +12 volt power rail and node 306 and a P-FET
376 coupled between the +12 volt power rail and second node 308
through a sense resistor 377. The H-Bridge FET construction of VCM
driver 92 also includes an N-FET 378 coupled between the ground
node and first node 306 and an N-FET 380 coupled between second
node 308 and the ground node through sense resister 377.
VCM driver 92 includes a current sensor 379 and a pulse width
modulated (PWM) current controller 319. Current sensor 379
generates a current sense signal 93 that represents current flowing
through sense resister 377 and coil 70. Current controller 319
generates control signals 95a-95d in response to current sense
signal 93 and the analog signal representing desired VCM coil
current on line 91.
Current controller 319 controls which of FETs 374, 376, 378, and
380 are turned on via lines 91a, 91b, 91c, and 91d respectively.
The FETs of VCM driver 92 are controlled such that P-FET 374 and
N-FET 380 are turned on while P-FET 376 and N-FET 378 are off to
thereby conduct current from the +12 volt power rail to first node
306 and to ground node 308. Similarly, while P-FET 376 and N-FET
378 are turned on, P-FET 374 and N-FET 380 are off to thereby
conduct current from the +12 volt power rail to second node 308 and
to ground node 306. By switching selectively between the above two
states current flows through VCM coil 70 to cause movement of coil
70 which in turn causes attached actuator arm 62 to swing and
thereby move transducer head 64 over disk 50.
According to one embodiment, VCM voltage limiting circuit 302 of
disk drive 30 comprises a diode 320, a NPN bipolar transistor 322,
a resistor 324, and a resistor 326. Diode 320 is coupled between
node 206 and a collector of bipolar transistor 322. An emitter of
bipolar transistor 322 is coupled to node 208. Resistor 326 is
coupled between a base of bipolar transistor 322 and a node 328.
Resistor 324 is coupled between the +12 volt power rail and node
328.
VCM voltage limiting circuit 302 of disk drive 30 also comprises a
diode 330, a NPN bipolar transistor 332, a resistor 334, and a
resistor 336. Diode 330 is coupled between node 308 and a collector
of bipolar transistor 332. An emitter of bipolar transistor 332 is
coupled to node 306. Resistor 336 is coupled between a base of
bipolar transistor 332 and a node 328. Resistor 334 is coupled
between the +12 volt power rail and node 328.
The VCM velocity control signal from servo controller 88 is
provided on line 304 to node 328 to control the bases of bipolar
transistors 322 and 332. After servo processor 86 commands servo
controller 88 to temporarily activate the VCM velocity control
signal on line 304 two conditions exist in VCM voltage limiting
circuit 302. In the first condition, VCM driver applies the voltage
V to first node 306 and grounds second node 308 through sense
resister 377 causing diode 320 to be forward biased and diode 330
to be reversed biased, and bipolar transistor 322 goes from an off
state to a saturated condition which conducts current away from VCM
60 to limit the voltage V applied across coil 70 of VCM 60 to the
predetermined VCM voltage level between first node 306 and second
node 308 in order to limit the actuator arm velocity, such as
maintaining the movement of the actuator arm at a substantially
constant actuator arm velocity. In the second condition, VCM driver
applies the voltage V to second node 308 through sense resister 377
and grounds first node 306 causing diode 330 to be forward biased
and diode 320 to be reversed biased, and bipolar transistor 332
goes from an off state to a saturated condition which conducts
current away from VCM 60 to limit the voltage V applied across coil
70 of VCM 60 to the predetermined VCM voltage level between first
node 306 and second node 308 in order to limit the actuator arm
velocity, such as maintaining the movement of the actuator arm at a
substantially constant actuator arm velocity.
Suitably, the predetermined VCM voltage level across coil 70 of VCM
60 is the voltage drop from first node 306 to second node 308 which
is equal to the Vce of bipolar transistor 322 plus the Vfwd of
diode 320 for the first condition where diode 320 is forward biased
and is equal to the Vce of bipolar transistor 332 plus the Vfwd of
diode 330 for the second condition where diode 330 is forward
biased. In one example embodiment the Vce of bipolar transistors
322 and 332 is approximately 0.2 volts and the Vfwd of diodes 320
and 330 is approximately 0.7 volts resulting in a total voltage
drop from first node 306 to second node 308 of approximately 0.9
volts.
Limiting the voltage applied across coil 70 to the predetermined
VCM voltage level reduces the actuator arm velocity to a
substantially constant actuator arm velocity. Actuator arm 62
decelerates until the BEMF of VCM 60 is equivalent to the
predetermined VCM voltage level between first node 306 and second
node 308. At this point, actuator arm 62 is moving at a
substantially constant and safe actuator arm velocity toward outer
diameter crash stop 118. The safe substantially constant actuator
arm velocity corresponds to the predetermined VCM voltage level
between first node 306 and second 308 and can be ascertained by
knowing the BEMF factor (Ke) of VCM 60 and the voltage Vcoil across
coil 70 as shown by the above Equations I.
Alternatively, VCM voltage limiting circuit 302 does not include
diode 320 or diode 330 to thereby provide only the Vce of bipolar
transistors 322 or 332 between nodes 306 and 308 once the VCM
velocity control signal on line 304 is activated. This alternate
embodiment of VCM voltage limiting circuit 302 is suitably employed
in applications which require a reduced substantially constant
actuator arm velocity or where the BEMF factor (Ke) of VCM 60 is
quite low.
According to another alternate embodiment of VCM voltage limiting
circuit 302, diodes 320 and 330 are replaced with zener diodes or
resistors are coupled in series with diodes 320 and 330 to increase
the voltage drop between nodes 306 and 308 once the VCM voltage
control signal on line 304 is activated. This alternate embodiment
of VCM voltage limiting circuit 302 is suitably employed in
applications which require an increased substantially constant
actuator arm velocity once the VCM velocity control signal 304 is
activated or where the BEMF factor (Ke) of VCM 60 is quite
high.
The above described VCM voltage limiting circuit 302 is suitably
implemented in TTL logic using bipolar transistors for an
embodiment of disk drive 30 where VCM voltage limiting circuit 302
is implemented in discrete logic on disk controller circuit board
32. An alternative embodiment of disk drive 30 implements a VCM
voltage limiting circuit similar to VCM voltage limiting circuit
302, but in CMOS logic using FETs in one of the gate arrays mounted
on disk controller circuit board 32.
* * * * *